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Show 1894.] MYOLOGY OF RODENTS. 291 gurus, and presumably in Erethizon dorsatus and epixanthus, as it is not mentioned by Mivart1 or Windle2. In the Chinchillidae it rises from the upper part of the fibula, the p. quinti being absent (Chinchilla, Lagostomus). In the Dasyproctidae it rises from the whole outer surface of the fibula in Dasyprocta, in which there is no p. quinti; in Ccelogenys it only rises from the lower third of the bone. In the Caviidae it resembles the Chinchillidae in Ceredon and Cavia cobaya. Beddard mentions that it is present in Dolichotis. It is always present in the Sciuromorpha, having the usual attachments. In Sciurus, Pteromys, Xerus, and Spermophilus it rises from the lower quarter of the fibula and runs to the fourth digit. In Arctomys marmotta it sent an additional slip to the third toe. In Castor it wTas joined by a small muscular slip from the calcaneum, probably part of the extensor brevis digitorum. It will be noticed that the only animals in which this muscle was wanting were the Tree-Porcupines. Peroneus Quinti Digiti.-The p. quinti when it is present rises from the outer surface of the fibula above the last muscle, and is inserted into the extensor longus tendon on the dorsum of the fifth toe. It is present in the Octodontidae (Myopotamus, Capromys, Octodon, Aulacodus), in the Hystricidae (Hystrix, Sphingurus, Erethizon), in Ccelogenys, and in all the Sciuromorpha examined. It is absent in Dipus cegyptius, in the Chinchillidae (Chinchilla, Lagostomus), in the Caviidae (Cavia cobaya, Ceredon, Dolichotis), and in Dasyprocta. The presence or absence of the p. quinti seems to depend entirely on the degree of development of the fifth toe. It is not nearly as persistent a muscle as the extensor proprius hallucis, which is so often found when no hallux exists ; it seems indeed to precede the disappearance of its toe, because in Chinchilla the muscle is wanting, although there is a small fifth toe. Gastrocnemius.-The gastrocnemius rises by two heads from the upper and back part of the two condyles, fabellae often being present. The two heads unite with the soleus to form the tendo Achillis. The fibres of this tendon are twisted so that those that are derived from the inner head of the gastrocnemius become superficial and eventually external. In Castor canadensis the two heads remain separate as far as their insertion. The presence or absence of the fabellae does not seem to depend on the affinities of the animal, as they are large in Aidacodus on both sides, while in Myopotamus only the outer one is present. In Dasyprocta they are both present, in Ccelogenys both absent. In the Sciuromorpha, however, they were found in every case except that of Castor canadensis (Sciurus, Pteromys, Xerus, Spermophilus, Arctomys, Castor). Soleus.-The soleus rises in all cases from the posterior aspect 1 P. Z. S. 1882, p. 271. 2 Journ. Anat. vol. xxii. p. 126. |